A well-known thermal fixing device includes an endless fixing (fusing) film, a heater disposed in an internal space of the fixing film, a pressure roller that nips the fixing film together with the heater, and a pair of guide members that guides an outer circumferential surface of the fixing film. The guide members are disposed respectively on both widthwise ends of the fixing film. Developer on a recording sheet is thermally fixed thereon while the recording sheet is conveyed between the fixing film and the pressure roller (a nip region). Each guide member has a cylindrical shape whose inner circumferential surface serves to guide the outer circumferential surface of the fixing film.
Another conventional fixing device is provided with a pair of guide members having outer and inner guide portions for guiding an endless fixing film. The outer guide portion serves to guide an outer circumferential surface of the fixing film, while the inner guide portion serves to guide an inner circumferential surface of the fixing film. In the fixing device, the outer and inner guide portions are formed to have a protruding length identical to each other.